1. Field of the Invention
This invention relates to a recording or reproducing apparatus and more particularly to a device provided in the apparatus for mounting a recording medium on a rotary driving shaft.
2. Description of the Prior Art
There are various known recording or reproducing apparatus which are arranged to record or reproduce information on or from a recording medium such as a rotary magnetic sheet. The apparatus of this kind must have the recording medium accurately and reliably mounted on a rotary driving shaft. Inadequate mounting results in the eccentricity of recording tracks relative to the axis of rotation. Then, in reproducing the signal recorded on the recording medium by a reproducing apparatus other than an apparatus used for recording, for example, the eccentric recording tracks become hardly traceable. Further, inadequate mounting causes an increase in a spacing loss between the recording medium and a recording or reproducing head. The increased spacing loss then comes to hinder adequate signal conversion to be performed between the recording medium and the head.
To avoid these inconveniences, there have been contrived various methods for lessening the eccentricity including an arrangement to press fit the recording medium on a rotary driving shaft, an arrangement to prepare the rotary driving shaft in a tapered shape, etc. The conventional mounting methods further include the provision of positioning means for setting the recording medium in a predetermined position by providing a permanent magnet on the rotary driving shaft with a magnetizable member arranged in the corresponding position on the recording medium. Then, by virtue of an attraction between the two, the recording medium can be set in the predetermined position in relation to the rotary driving shaft. In another known method, pressing means is arranged to forcefully push the middle part of the medium against the driving shaft to ensure that the medium and the shaft rotate in one unified body or in firm contact with each other during a recording or reproducing operation.
In the method of using the permanent magnet, the magnet must be sufficiently large in size for obtaining a required magnitude of attraction or must be made of an expensive rare earth material. It is another disadvantage of this method that, in the case of such a recording medium as a magnetic sheet, the information recording or reproduction might be affected by the increased attraction of the permanent magnet. Therefore, there is an increasable limit to the attraction. In the case of the method of using the pressing means for forcefully pushing the recording medium against the rotary driving shaft to have the medium rotate in one unified body with the shaft or to keep them in firm contact, the rotating precision of the medium tends to be affected and the load on a driving motor is increased by the action of the pressing means. That method is, therefore, not suitable for a recording or reproducing apparatus using a small rotary magnetic sheet or a small motor.
To solve these problems, there have been proposed some apparatuses arranged to push the mount part of a recording medium against a rotary driving shaft in a press fitting manner in mounting the medium and, after that, to cancel the press fit state. In one example of such arrangement of the prior art, a recording medium receiving member which is movable for bringing the medium in a recording or reproducing position is provided with a movable external cover member for covering the receiving member. The external cover member is arranged to bring about the above-stated press fit state when it moves and to automatically release the press fit state when it is locked to the body of the apparatus in a given position. In another example of such arrangement, the apparatus is provided with some release means that is arranged to undo the above-stated press fit state in response to completion of the covering action of the external cover member. In both cases, the press fit mounting means is arranged in association with the external cover member. Therefore, the arrangement does not ensure adequate positional precision in pressing the mount part of the recording medium. Besides, it presents a difficulty in terms of assembly precision. It also tends to become large in size because the pressing action must be performed on a recording medium in its state of being placed within the receiving member. The arrangement, therefore, tends to hinder an attempt to reduce the size of the apparatus.